Well, this is somewhat complicated and we wouldn't want to give you non-authoritative information on this.
Typically, you would close the last plan on your account by closing your account. That is not to say that your account would actually be totally closed because you'd still have access to the domain registration part of your panel (and thus in some sense your account would still be active).
You would not really have a "hosting account", though.
This would obviously also end your hosting, which you don't want to do.
So normally you would just remove autobilling from your account. But since you will have an active rewards account, your payment would be automatically drawn from your rewards balance.
All this to say that your situation is complicated enough that I recommend contacting support.
Remember, most of the people here on the forums don't have a lot of experience with closing their accounts because... we all actively use dreamhost for our hosting. That why we're here, right?
Particularly in this forum, you'd normally also find readers who were thinking of opening an account here at dreamhost. These folks also wouldn't have a lot of experience with closing their hosting account. If you don't get good answers here, you might think about posting your question in a more appropriate forum, though as I said earlier, your best bet may be sending in a low priority support ticket.
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